KIPP KEY Academy in Washington, D.C. North Star Academy in Newark. Roxbury Prep in Boston. Amistad Academy in New Haven. These, and perhaps two hundred other high-performing schools nationwide, are the bright lights of the charter school movement. Despite social and economic disadvantages, their students not only trounce their district peers on state tests but also top statewide averages and, in some cases, surpass students from surrounding affluent suburban districts. Among these "gap-closing" schools, one broad approach, frequently called "no excuses" schooling, appears to dominate. The Knowledge Is Power Program (KIPP) network of schools is the exemplar, but this approach is proliferating in other networks, including Achievement First and Uncommon Schools, and in stand-alone schools, many of which aspire to replicate themselves in coming years. But to narrow America's shameful achievement gaps, we would need thousands more such schools. Is the "no excuses" approach sustainable, and can it be widely reproduced? To explore this question, the author turned to a locale well known for its concentration of strong charter schools, Boston, and examined whether the successful charter schools in Boston rely on privileged inputs of students or of financial or human capital. (Contains 18 notes.)